This invention relates to signal transmitters and, more particularly, to apparatus for optimizing the performance of a power amplifier in a signal transmitter.
Power amplifiers are used in communication systems, such as signal transmitters or transceivers, for amplifying signals to be transmitted. For example, power amplifiers are used in radio transmitters, cellular telephone transmitters, personal communication systems (PCS) and the like. In a typical signal transmission application, such as in a PCS, the signal to be transmitted, such as a speech signal, is modulated onto an RF carrier and then is transmitted from, for example, a cellular telephone to a base station. During a typical transmission session, as when speech or digital signals are transmitted by the RF carrier, the power amplifier output level varies over a range from a minimum output level to a maximum output level. As a numerical example, the output power generated from the power amplifier may vary from practically zero to an output power level on the order of about 600 milliwatts (mw).
For optimal power consumption by the power amplifier, the load impedance thereof should be matched to the power output level. Heretofore, it had been thought that it would be quite difficult and expensive to couple a variable load impedance to the power amplifier and to control that load impedance such that it is matched to the amplifier output impedance, yet varies as a function of the power amplifier output level. Hence, it has been common to fix the power amplifier load impedance at a predetermined value which, for the most part, is expected to match the power amplifier output level within an accepted tolerance range. However, when the output power generated by this amplifier is outside the range for which the load impedance has been designed, power consumption by the power amplifier may be much greater than necessary, thereby exerting a significant drain on the battery which may be used as the power source.